Upside of booking your companion(s) separately is that as an MVP, you won't be upgraded to Premium or First Class with a companion(*), and unless you get an agent willing to bend the rules, won't have access to non-Premium elite seats until OLCI.

Upside of booking your companion(s) with you is they will have the same elite baggage allowance, & can be seated in non-Premium elite seats, and will be handled together automatically in the event of IRROPs. You can use the link reservations on the website but this just places a note on the reservation more or less and doesn't actually merge the two together.

You can always split the reservation after booking, but your time of First Class upgrade request would be at that point. I've also had an agent say the computer wouldn't let her leave my non-elite companion in an Elite seat after splitting.

The other advantage to purchasing a trip as a single RT, other than what EmailKid mentioned is if something happened to the other flight(s) such a travel waiver or IRROPs, it would be easier to cancel the whole thing vs two separate one-ways if only one leg was affected.


(*there have been multiple exceptions to this reported in other threads, but it appears to be route/load dependent and is totally at Alaska's discretion)